1. Field of the Invention
The present invention relates to glass compositions which are suitable for being converted into a glass ribbon from which panes can be cut, and which resist heat well. These panes can be used to make fire-resistant glazing or can serve as substrates for the manufacture of plasma screens, electroluminescent screens and cold-cathode screens (field-emission displays).
2. Discussion of the Background
The glass typically used to make substrates of this type is a glass belonging to the family of silica-soda-lime glasses commonly used to manufacture glazing intended for buildings or motor vehicles. Although this type of glass is generally satisfactory in terms of its chemical stability, planarity and the defects which it has, its thermal stability sometimes leaves something to be desired.
During the manufacture of the emissive screens of the plasma screen type, the substrate is subjected to a number of heat treatments which are intended to stabilize the dimensions of the said substrate and fix a series of layers of different compounds, for example enamels, deposited on its surface. In order to fix these layers, the thicknesses of which may vary, it is necessary for the substrate to be heated to temperatures in excess of 550.degree. C. Although the coefficient of expansion of the silica-soda-lime glass which is used is of the same order of magnitude as that of the compounds deposited on its surface, its thermal stability is insufficient and it is necessary to place it on a rectified slab during the heat treatments in order to prevent any deformation.
The glasses used for manufacturing fire-resistant glazing generally belong to the borosilicate glass family. These glasses, which resist heat and thermal shock very well, are generally characterized by a low coefficient of expansion. The latter characteristic makes it impossible for high stresses to be developed in these glasses by thermal toughening, which limits the extent to which their mechanical strength can be increased in this way.
New families of glass compositions have therefore been developed and described in patent WO-96/11887, in order to overcome these drawbacks, in particular in order to make it possible to manufacture panes or substrates having substantially no deformation during heat treatments of the order of 550 to 600.degree. C. and capable, through thermal toughening, of exhibiting stress levels comparable with those obtained with standard silica-soda-lime glass.
One family of glass compositions which is particularly advantageous for a plasma screen application described in the aforementioned patent is that which uses little or no alumina (Al.sub.2 O.sub.3), a high level of zirconia (ZrO.sub.2) and very specific alkaline-earth metal oxide contents, including barium oxide (BaO).
However, in a way which has not yet been explained, it has been found that, in some cases, substrates or panes subjected to customary treatment cycles, in excess of 500.degree. C., can develop optical defects which, in particular, are manifested by local colorations, and these defects persist after washing. This optical degradation impairs the production efficiency.